1. Field of the Invention:
This invention pertains to stereoscopic viewing systems in general, and in particular, to a high efficiency, stereoscopic display capable of producing color images.
2. Description of the Related Art:
An observer with normal binocular vision sees a slightly different perspective of a real-world, three-dimensional scene with each eye. This relative perspective shift between eyes is a function of the scene object distance and results in a perception of depth in the scene by the observer.
Over the years, various techniques have been developed to present a three-dimensional scene to an observer utilizing a two-dimensional format. These typically involve the display, either simultaneously or time-sequentially, of a pair of two-dimensional images corresponding to right-eye- and left-eye-perspective views of a three-dimensional subject, which images are each "encoded" with right-eye or left-eye information that permits them to be seen only by an eye equipped with, or shuttered by, an appropriate "decoder".
A well-known example is the "stereoscope" viewer, known since Victorian times, which first records a pair of static, two-dimensional images, usually photographs, corresponding to right-eye and left-eye perspectives of a three-dimensional scene in fixed, side-by-side relationship, and then presents them for viewing through apparatus which confines the vision of each eye of the viewer to the appropriate, corresponding image using optics. An improved form of this device is available today in most toy shops.
Another technique which has been used to present moving scenes, as in motion pictures, involves recording the right-eye and left-eye perspectives in different colors on the same frame of film, then viewing the projected frame through eyewear comprising a pair of color filters, each of which is disposed within the eyewear to pass the appropriate image to a corresponding right or left eye and to block reception by that eye of the other, simultaneously-presented, but differently-colored image.
A variation of this technique is to record the right-eye and left-eye perspectives on superposed sheets of orthogonally polarized dichroic polarizing materials, and then to view the perspectives projected simultaneously onto a non-depolarizing screen through eyewear having a pair of orthogonally polarized lenses. A description of the two foregoing techniques, along with a modification of them said to be more suitable for presenting sterescopic images for viewing on television, is presented in U.S Pat. No. 4,431,265 to Benton.
Wilkins, in U.S. Pat. No. 4,559,556, describes polarizing eyewear useable with a method of split-screen image presentation in which the right-eye and left-eye perspectives are presented simultaneously in an "over-and-under", orthogonally polarized fashion on a television screen for viewing.
In U.S. Pat. No. 4,583,117, Lipton et al. describe a similar "over-and-under" type of image presentation in which a pair of conventional videocameras are modified to utilize only half of the horizontal sweep lines to form the perspectives, which are then presented time-sequentially. Electronically shuttered and synchronized eyewear is employed to view the scenes without flicker.
"Shuttering" can be accomplished either mechanically or electro-optically. The latter technique is typically based on liquid crystal devices. A liquid crystal shutter consists of a sandwich of a first plane polarizer, an active liquid crystal device, and an "analyzer", i.e., a second plane polarizer. In one voltage state, the liquid crystal device is optically neutral, and in a second state, serves to rotate the plane of polarization of light transmitted through it by 90.degree.. By appropriate initial alignment of the axes of polarization of the three elements, the shutter can be thereafter electrically switched between "open" and "closed" conditions.
A pair of goggles employing two such shuttering devices over each eye and synchronized by means of a cable interconnecting them to a display of sequentially-presented right-eye and left-eye images is described by Roese in U.S. Pat. No. 4,021,846.
It is possible to split the shutter and to position the first polarizer and liquid crystal device in front of the image generating device. In such a configuration, the two elements, when switched synchronously with the sequential image display rate, operate as a "switchable polarizer" to polarize right-eye and left-eye images alternately in orthogonal directions, to be analyzed appropriately by the the second polarizer, worn by the observer as a pair of polarizing spectacles having orthogonal axes of polarization. Stereoscopic displays employing such a technique are discussed by Bonne in U.S. Pat. No. 3,858,001 and by Byatt in U.S. Pat. No. 4,281,381.
In U.S. Pat. No. 4,670,744, Buzak describes an entirely different means for displaying a three-dimensional image using a cathode ray tube ("CRT") to sequentially project a series of images corresponding to different depth planes of a three-dimensional subject. These images are then selectively transmitted or reflected from appropriate positions along an optical path by means of a plurality of "light direction modulators", which are notable for their use of "chiral cells", i.e., cholesteric liquid crystal cells, and circularly-polarized light, to achieve the desired reflections and transmissions.
It is to be noted that the foregoing art has timely application in the field of avionics, wherein visual information of a type useful to a pilot or electronics warfare operator, such as navigation, fire-control, or threat-environment information, can be displayed in three-dimensions and within the context of the pilot's or operator's three-dimensional view of the real world.
However, the prior art techniques discussed above present certain problems in terms of their practical utilization in an avionics system. For example, those which employ shutter elements in the operator's eyewear are bulky and clumsy to wear, have a low see-through transmission (less than about 20%), and must be tethered to the display by means of a cable or radio signal to carry the synchronizing signals.
Those which use colored eyewear also transmit relatively low levels of light and present a view of the real world that is both dim and strangely colored.
Even those which employ eyewear consisting of a simple pair of "clear", linear polarizers with crossed polarizing axes are not without such problems because, (1) the latter are still capable of transmitting only about 40% of the incident real world light to the operator's eyes, a level that is considered unacceptable in many avionics applications where high "real world" situational awareness must be maintained, and (2) it is difficult to keep the first polarizer element and the "analyzer" elements properly oriented with respect to each other, particularly when the observer's head is subject to being tilted sideways, as in a maneuvering airplane. Such disorientation can cause loss of shutter efficiency and create "ghosting" on the display in which the ghost image is seen by the "wrong" eye.
Displays which synthesize three-dimensional images from a plurality of depth planes lack sufficient definition unless a large number of such planes are generated, leading to very complicated and expensive displays.
The present invention teaches a way to overcome these problems in a three-dimensional display that is well suited to avionics applications and may be provided in either monochromatic or full-color embodiments.